configparser: write() silently drops leading whitespace from multiline values on round-trip #161317

Bug report

If a value contains a newline followed by whitespace, writing and reading it back silently loses the whitespace with no error.

import configparser, io

c = configparser.ConfigParser()
c['s'] = {'k': 'line1\n\tline2'}

buf = io.StringIO()
c.write(buf)

c2 = configparser.ConfigParser()
c2.read_string(buf.getvalue())
print(repr(c2['s']['k']))
# Got:      'line1\nline2'
# Expected: 'line1\n\tline2'

The written file has:

[s]
k = line1
line2

_write_section adds \n\t to mark continuation lines. On read, _handle_continuation_line calls .strip() on each line — which strips both the added tab AND any whitespace that was part of the original value.

On 3.14+, write() should raise InvalidWriteError here, same as it already does for keys with leading whitespace (gh-96239). On earlier versions the data is just silently lost.

CPython versions tested on:

3.13+

Operating systems tested on:

Linux

Linked PRs

• gh-161317: configparser.write() raises InvalidWriteError for multiline values with leading whitespace #161337